This invention relates to image rejection and enhancement mixers and in particular to image rejection and pseudo-image enhancement mixers utilizing only two diodes.
A common problem for wide open receivers is to differentiate between a signal of interest and its image, an unwanted input frequency that arises from a source other than that to which the receiver is tuned. In narrow-band communication receivers, one approach to the image problem is to insert a filter in front of the mixer. However, when the RF bandwidth stretches across several octaves, only an electronically controlled preselector, such as a YIG filter, can be used to differentiate the image frequency. In addition, these filters are relatively expensive, increase the receiver's noise figure and offer rather sluggish tuning speeds, making it difficult to continuously monitor frequency agile radar signals.
Another approach to image rejection involves the use of a channelized mixer. The benefits of an image channelized mixer are twofold: they help a receiver operator identify whether a signal is a true signal or its image, and they can be used to reduce the image noise generated by an RF amplifier.
Another important aspect of channelizied mixers is their ability to properly handle intermodulation product. In general, the mixer is a nonlinear device, thus it generates harmonics of the input signals. Without proper RF filtering, harmonics of one signal can mix with a second input signal to produce an in-band spurious IF response. Even with a single input signal, harmonics of the input signal can interact with harmonics of the local oscillator signal and produce harmonically related intermodulation products with the IF bandwidth.
It is known that the conversion loss of a mixer can be made to approach zero if all the harmonic and intermodulation frequencies can be reactively terminated and properly phased. Where f.sub.LO is the local oscillator frequency and f.sub.RF is the frequency of the input signal, each modulation product, mf.sub.LO .+-.nf.sub.RF, possesses some energy and represents loss unless converted back to IF frequency. It is practically impossible to properly control the impedances at each of the frequencies, especially when the mixer is to operate over a wide frequency range. Therefore, the primary aim is to reactively terminate and properly phase the (2f.sub.LO -f.sub.RF) intermodulation product, often incorrectly known as the image. If this pseudo-image cannot be well shorted or opened across the full band, then control of the other intermodulation products will do no good.
Although the (2f.sub.LO -f.sub.RF) intermodulation product frequency is identical to the frequency of the image, a major distinction exists, which has not been properly observed by many. The image is a potential, or in fact, an actual input signal. The (2f.sub.LO -f.sub.RF) pseudo-image is generated by the mixer diodes and, therefor, is not and logically cannot be an input signal to the mixer that generated it.
Channelized mixers are further considered below in connection with FIG. 1 and FIG. 2.